Asthma is a common chronic respiratory disease characterized by paroxysmal or persistent respiratory symptoms associated with variable airflow limitation and airway hyperresponsiveness. The early diagnosis and treatment of asthma is important for improving the health of the patient and minimizing the social and economic burden of the disease. No single symptom or diagnostic test defines asthma; it is a heterogeneous disease with a variety of symptoms, including wheezing, cough, shortness of breath, and chest tightness. International guidelines specify that asthma diagnosis should be based on both symptoms and objective evidence of variable airflow obstruction and/or airway hyperresponsiveness. The main diagnostic features are an obstructive pattern on spirometry, a positive bronchodilation test and evidence of reversibility or variability in peak expiratory flow (PEF) or spirometric results after treatment. Direct and indirect methods of revealing bronchial hyperresponsivenesss (BHR) and markers of inflammation, such as differential eosinophil count in induced sputum, exhaled nitric oxide (NO) and pH in exhaled breath condensate, are also considered key points in asthma diagnosis. Recently, small molecules generated from cellular metabolic activity, known as metabolomics, have been investigated as a potential diagnostic tool. The diverse features and phenotypes of asthma add complexity to the diagnosis, which should be made with caution using a reliable approach, in order to reduce the possibility of over- and under-diagnosis.